The present invention relates broadly to an electromagnetic interference (EMI) shielding gasket or seal having a resilient core and a conductive fiber mesh sheathing which is consolidated to mitigate its unraveling or splaying when the gasket is terminated to a determinable length.
The operation of electronic devices such as computers, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., from between about 10 KHz and 10 GHz, and is termed "electromagnetic interference" or "EMI" as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other "target" devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., "Gaskets That Block EMI," Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
Conventionally, the EMI shielding gaskets heretofore known in the art have involved a resilient core element having gap-filling capabilities, around which is provided a conductive, tubular sleeve or other sheathing. The resilient core element typically is formed of an elastomeric foam which may be a foamed elastomeric thermoplastic such as a polyethylene, polypropylene, polypropylene-EPDM blend, butadiene, styrene-butadiene, nitrile, or chlorosulfonate, or a foamed neoprene, urethane, or silicone. Alternatively, an unfoamed silicone, urethane, neoprene, or thermoplastic may be utilized in either a solid or tubular form. Resilient, thermoplastic elastomeric foams and methods for making the same are detailed in U.S. Pat. Nos. 5,393,796; 5,070,111; and 4,898,760.
The sheathing may be provided as a woven or non-woven fabric, or as a knitted mesh. Knitted meshes may be preferred from a manufacturing standpoint as eliminating such fabrication steps as slitting, wrapping, and adhering which are necessitated with the use of fabric sheathing. The fabric or mesh may be formed of a metal wire such as copper, nickel, silver, aluminum, tin or an alloy such as Monel, or of other conductive fibers such as carbon, graphite, or a conductive polymer. Alternatively, nonconductive natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyamide, or the like may be plated or otherwise coated with a conductive material such as metal, carbon, or the like. Depending upon the needs of the specific application for the seal, a combination of conductive and nonconductive fibers may be used. Preferred sheathing materials include nylon or polyester yarn plated with silver, copper, nickel, or tin.
A representative EMI shielding gasket which includes a resilient foam core within a conductive sheath is disclosed in Buonanno, U.S. Pat. No. 4,857,668. The sheath is bonded to the foam core as an integral part of a continuous molding process in which the foam is blown or expanded within the sheath. As the foam cures, a sealed outer boundary layer forms on the surface thereof which faces the inner surface of the sheath. The outer boundary layer is stated to have an adhesive character which effects a strong bond between the foam core and the sheath.
Buonanno, U.S. Pat. No. 5,202,536, discloses an EMI seal having an elongated, resilient core which is covered with a partial conductive sheath. A conductive portion of the sheath, preferably a metallized fabric or the like in a resin binder, is provided to extend partially around the core to define ends which are non-overlapping. A second, nonconductive sheath portion is attached to the core element to extend between the ends of the conductive sheath portion. The seal may be produced in accordance with the continuous molding process described in U.S. Pat. No. 4,857,668. Alternatively, the core may be pre-molded with the conductive and nonconductive sheath portions being attached thereto with an adhesive or the like.
Keyser et al., U.S. Pat. No. 5,028,739 discloses another EMI shielding gasket which includes a resilient, elastomeric core enveloped within a fine, open format knit or braided wire mesh. An adhesive strip is disposed lengthwise along a surface of the gasket which allows the gasket to be removably fastened directly to an enclosure.
Matsuzaki et al., U.S. Pat. No. 5,142,101, discloses yet another EMI shielding gasket having a resilient core to which a metal mesh is adhered with a rubber macromolecule adhesive or the like. The core includes a body portion for contact with a first surface of a joint of a conductive housing, and an installing portion which may be inserted into a second surface of the joint. A segregating portion is formed between the body and installation portions which separates the metal mesh from the surface of the core such that the mesh is not broken as the core is elasticly deformed during its insertion into the joint.
Although gaskets of the type herein involved have represented important advancements in the art of EMI shielding, a problem has continued to plague the industry. Namely, it has been observed that when gaskets of such type are sectioned or otherwise terminated to length for mounting within the boundaries of an interface to be sealed, the sheathing at the terminal end or ends of the gasket may have a tendency to splay or unravel. Apart from deleteriously affecting the conductivity of the sheathing, the unraveling of the conductive fibers of the sheathing may cause shorting within the electrical circuitry which is enclosed within the housing being sealed. Gaskets having a sheathing of a relatively open, interlocked knit or braided structure are particularly prone to unraveling at the terminal ends thereof, with gaskets having a sheathing of a tighter, woven fabric being more stable.
Indeed, it is known to attach a fabric and other sheathing to a foam core with a pressure sensitive adhesive or the like. However, with respect to sheathings such as knitted fiber meshes having a relatively more open structure, such adhesives are not acceptable as having a tendency to migrate or bleed through the loops of the knit while under an applied pressure. The performance of the gasket thereby may be affected as the extraneous adhesive may interfere with the removability of a mating surface therefrom. Another shielding gasket manufactured by Instrument Specialties, Inc., of Delaware Water Gap, Pa., includes a bead of an unfilled, organic polymer between a first and second layer of sheathing fibers to effect a bond therebetween. Such technique, however, necessitates a complex sheathing weave which cannot be attained with conventional circular knitting machines commonly used to form knit sheathings.
The EMI shielding industry has otherwise responded to the problem associated with the sectioning of shielding gaskets by providing an outer "tie-coat" on the shielding. For example, a gasket marketed under the name Soft Shield II.TM. by Chomerics, Inc., of Woburn, Mass., incorporates an outer coating of a silicone or the like for maintaining the integrity of the terminal ends of the gaskets. Such coating, however, must be applied in a separate processing step which increases the costs of manufacturing the gasket.
Thus, the sectioning to length or other termination of the EMI gaskets and seals heretofore known in the art has generally lead to unacceptable conditions, or has necessitated the involvement of a separate manufacturing step. It therefore will be appreciated that improvements in EMI shielding gaskets and seals, and in the methods for terminating the same, would be well-received by electronics manufacturers and other industries.